Morphological and ultrastructural changes in Lactobacillus plantarum B21 as an indicator of nutrient stress

Abstract

This study investigated the effect of nutrient stress on cell viability and growth, bacteriocin activity, acid production, morphology and ultrastructure of Lactobacillus plantarum B21. The aim was to ascertain if this species’ viability in food applications could potentially be controlled/improved by initial culture conditions. Both glucose and Tween 80 were found to have statistically significant effects on cell viability, as well as pH and bacteriocin production. Scanning and transmission electron microscopy revealed that nutrient availability was also associated with changes in cell size and shape. Control cells displayed a regular, smooth, rod-shaped morphology with well-defined cytoplasmic contents, equal distribution of nuclear material and an intact cell-wall. In contrast, nutrient-stressed cells became irregular, varied in size and shape and had a ruffled, but intact, cell membrane with dense, fibre like protrusions. The absence of glucose in growth media resulted in a coccoid cell shape, whereas the absence of Tween 80 resulted in the formation of filamentous, rod shaped cells. This study proposes nutrient levels could be used as means of controlling the length to diameter ratio and shape of Lactobacillus plantarum B21 in order to produce robust, functional bacterial cells that could be used for applications in food technology.